JP2004511845A - PLC executed by an integrated web server - Google Patents

Abstract

A programmable logic controller for use as part of an industrial control system or as part of an automated system and a corresponding method, the controller including an interface to the Internet, and including a web server allowing a remote computer to access web pages maintained by the controller providing information relevant to the control function of the controller such as control sensor readings and, optionally, information about the status of the control system. The web server is implemented as part of the controller in such a way that the controller scan rate (the rate at which the ladder logic for all associated control system devices is repeated) is unaffected by execution of the web server.

Description

[0001]
(Field of the Invention)
The invention relates to the field of industrial control or automation. More particularly, the present invention relates to a programmable logic controller (PLC) for performing control functions associated with an industrial control or automation system, monitoring data associated with the control functions and monitoring general control system information. The present invention relates to communication through a network with a computer used for the communication.
[0002]
Industrial control systems often include a programmable logic controller (PLC) to coordinate control of industrial control equipment. The control equipment includes various elements, often either sensors that provide input to the PLC or relays that receive output from the PLC, each operating under the control of an element controller, Each is connected to a PLC via a network and a network I / O device. Industrial control using PLC requires what is called high-speed scanning. This implies a continuous fast execution of the next three main steps, which are repeatedly executed by the PLC. That is, the status of each input to the PLC required to execute the so-called ladder logic for the process to be controlled is grasped, the ladder logic for determining each output is clarified, and the status of the output is updated. .
[0003]
The term ladder logic is used to describe how the control elements of an industrial control system should be controlled based on the monitoring elements of the industrial control system in a manner that is intelligible to novice workers in the field of machine control. ing. The term ladder is used because the representation of the control logic is often in the form of a ladder, where each side of the ladder outputs, ie, a value and one for the state required of the control element. It has a value corresponding to the above input, that is, a signal from the monitoring element.
[0004]
For effective industrial control, the PLC performs a scan of the connected I / O devices at a constant scan rate, and gets too caught up in the surrounding tasks, and its regularly scheduled I / O The monitoring of the / O device must not be neglected.
[0005]
Typically, process operations are monitored at least intermittently at one or more central control stations by supervisory personnel. Each station samples the status of the operator-selected PLCs (and their associated sensors) and presents the data in some meaningful format. The management station may or may not be located at the same site as the monitored device, and often a single central station can access multiple sites (whether they perform the relevant processes or not). Does not matter). Thus, even in a traditional industrial environment where process equipment is in physical proximity, the communication chain can be extremely important. Because at least some supervisors may be absent.
[0006]
The PLC and the associated monitoring stations are connected by a computer network to facilitate the necessary communication. Typically, the network is organized so that any computer can communicate with any other network computer. The mechanism provided by the communication protocol allows the message to be broken up and sent to the destination computer specified by some form of address. The protocol can place a "header" of routing information in each component of the message, which specifies the source and destination addresses and identifies the component so that the destination computer can later play the entire message. Becomes easier. This data transfer technique allows the network to handle large volumes of communication quickly and efficiently without slowing down the transfer rate or requiring each network computer to process each network message, and to accommodate long individual messages. can do. The degree of routing depends on the size of the network. Each computer in the local network typically examines the header of each message and finds a match with the computer's identifier. Multi-network systems use routing information to first derive message components to the proper network.
[0007]
PLCs have been interconnected by computer networks for some time. See, for example, U.S. Patent No. 5,307,463. In a typical system, the monitoring computer is located remotely from any or all PLCs, accesses and periodically queries the PLC, and stores data describing the process or machine to be controlled or the PLC itself. get. These data can then be used for analysis by a monitoring computer.
[0008]
U.S. Pat. No. 5,805,442 discloses what is referred to as a distributed interface. This causes a remote computer to obtain information from the PLC over the Internet, and the information obtained from the PLC includes both data and instructions on how to display this data (ie, the term "distributed interface" is used). This is because at least some of the instructions for displaying data from the PLC are located on the PLC, not on the remote computer, and are transmitted to the remote computer along with the data to be displayed). The PLC disclosed herein incorporates a web server, a module that provides data and instructions to display the data in response to a data request received over the Internet. The combination of data and display instructions resides on one or the other PLC storage as so-called web pages. However, no logic is disclosed that allows the PLC to keep the scan speed of the I / O device attached to it constant.
[0009]
There is a need for a method of incorporating a web server into a PLC that does not affect the PLC scan speed by responding to data requests from the PLC by a remote computer.
[0010]
(Summary of the Invention)
Accordingly, the present invention provides a controller and corresponding method as part of an industrial control or automation system. The controller controls one or more input / output devices, and the controller can interact with a remotely located computer through a network such as the Internet. The controller includes means for performing a controller scan including solving ladder logic associated with at least one controlled input / output device, wherein the controller scan has a predetermined scan speed, and A web server responsive to a data request from a remotely located computer, the web server comprising data and associated instructions retrievable by a remotely located computer. A web page, wherein the instructions cause a remotely located computer to display data in a predetermined format, and wherein the web server responds to the data request with one or more web pages. And display the requested data in it.
[0011]
In another aspect of the invention, the means for collecting data executes as a background task.
In yet another aspect of the invention, the means for collecting data executes in parallel with the means for performing a controller scan.
[0012]
In yet another aspect of the invention, in response to a data request from a remote computer, the means for collecting data comprises a combination of data and instructions less than a predetermined maximum number of bytes per scan by the means for performing a controller scan. Supply. In some applications according to this further aspect of the invention, the maximum number of bytes per scan is less than the time required for a single scan in the means for performing a controller scan to a network for delivery to a remote computer in less than the time required for a single scan. The number of bytes that can be transmitted is predetermined.
[0013]
In yet another aspect of the invention, at least a portion of the data resides on a different controller, and the data collection means is configured to access the different controller and obtain the data over a computer network. .
[0014]
In yet another aspect of the invention, the controller comprises a Transmission Control Program (TCP) / Internet Protocol (IP) stack customized to convey any MODBUS command or message as a single TCP / IP packet. I have. In some applications according to this further aspect of the invention, the web server comprises a file server serving the web page and a hypertext transfer protocol (HTTP) server with file retrieval functionality. Also, some applications include a file transfer protocol (FTP) server that receives new or exchanged web pages over a network via a TCP / IP stack and provides them to a file server.
[0015]
The foregoing and other objects, features and advantages of the invention will be apparent from a consideration of the following detailed description, taken in conjunction with the accompanying drawings.
[0016]
(Best mode for carrying out the invention)
Referring to FIG. 1, a programmable logic controller (PLC) 10a (hereinafter simply referred to as a controller) for use as part of an industrial control system or as part of an automation system, according to the present invention, has a ladder scanning functionality. And a web server module 12. Module 11 with ladder scanning functionality activates I / O module 22 connected to device 23. The device 23 is either a sensor or an actuator, and solves what is called ladder logic, and determines an output that commands the next state of the device based on all inputs to each of the linked devices. Devices controlled by the PLC can be attached to the controller directly (like device 23a), by local network 60 (like device 23b) or by the Internet 70 (like device 23c). For device 22c, which is connected to controller 10a by the Internet 70, the controller uses the Internet interface 30a to process inputs and outputs associated with device 22d. The network interface includes a MODBUS handler 31 in a Transmission Control Program (TCP) / Internet Protocol (IP) stack 33. The MODBUS handler 31 has a certain MODBUS functionality and is provided for performing communication via the Internet by TCP / IP.
[0017]
The term MODBUS here refers to a simple, vendor-independent communication protocol system for supervision and control of automation equipment. In a preferred embodiment, the TCP / IP stack is MODBUS / TCP compliant. MODBUS / TCP is one of the MODBUS protocol systems, and corresponds to the use of MODBUS message transmission in the “Internet” or the “Internet” environment using the TCP / IP protocol. The most common use of the TCP / IP protocol at this time is when the controller, I / O module and "gateway" are attached to another simple field bus or I / O network over Ethernet. It is preferable that any firmware of the Internet interface 30a is based on MODBUS TCP / IP defined by Open MODBUS / TCP specification, release 1.0. The contents of this document are hereby incorporated by reference. This specification defines how MODBUS commands and responses are delivered to and from the MODBUS server over the Internet using the well-known port 502.
[0018]
In the preferred embodiment, TCP / IP stack 33 is a custom stack, programmed to provide any MODBUS command or message as a single packet, optimizing all MODBUS communications.
[0019]
The web server 12 communicates hypertext with a file server 20, preferably using a linked list file system, in accordance with a so-called HTTP server 32, the Internet Hypertext Transfer Protocol THHP (web page browser). And used by browsers to display web pages). In one embodiment, the HTTP server 32 has minimal functionality, such as only "file get" functionality. In the preferred embodiment, web server 12 also includes a file transfer protocol (FTP) server 34 that accepts downloads of new and exchanged web pages and provides them to file server 20.
[0020]
The TCP / IP stack 33 determines whether the incoming message (TCP / IP packet) is addressed to the MODBUS handler 31, the HTTP server 32, or the FTP server 34 based on the port number specified in the incoming message. I do.
[0021]
Referring to FIG. 2, a user of a remote computer 50 uses a browser 52, having a graphical user interface displayed on a display 51, through a network interface 53 through the Internet 70 to control the controller 10a and its Internet interface 30a. When communicating via the HTTP server 32, the controller operates as a website server for the browser 52, and via the HTTP server 32 and the file server 20, the controller 10a transmits the web page stored in the storage device. 21 is transmitted to the browser 52. The web page includes both the data provided by the module 11 having the controller and ladder scanning functionality and instructions on how the browser 52 displays the data. In general, web page 21 can include data provided by other controllers as well as data from devices attached directly or indirectly to host controller 10a. Thus, the first controller 10a may include data provided by the second controller 10b on its web page. The data provided by the second controller is, for example, data relating to a device attached directly or indirectly to the second controller. The web page may include data from other controllers as embedded data or as a reference to the location of the other controller's data, and the reference is understandable to the browser 52 so that the browser has such a reference. When a web page is loaded, a browser can be processed to obtain data referenced from other controllers.
[0022]
Please refer to FIG. According to the present invention, the ladder scanning task performed by the module 11 having ladder scanning functionality and the web server task performed by the web server 12 are performed in parallel by the controller 10a in a single central processing unit (CPU). Runs as a thread. The term parallel thread is used herein to refer to the case where a single central processing unit (CPU) has a parallel processing function, the CPU executes a ladder scan task in parallel with a web server task, or a standard thread. For a sequential processing CPU, it is assumed that the web server runs on a small portion of the same time slice as compared to the portion allocated for performing the ladder scan task for any given time slice. . That is, the web server is executed as a background task.
[0023]
As shown in FIG. 3, the ladder scanning task includes executing ladder logic, performing input / output processing for each attached device, and assigning any controller to a controller command. including. These three tasks are repeated many times at the scan speed referred to herein, and according to the present invention, the scan speed is not affected by the execution of the web server task. As shown in FIG. 3, the web server task includes a check for a request for port 80 (a request for an HTTP server), answering the request if there is one, but answering a request for a message longer than a predetermined length. Absent. The predetermined length of the message is such that the background task can process the predetermined length message in less than the time required for the CPU to complete one ladder scan. If the message is longer than the predetermined length, the web server processes a portion of the message in one scan and processes a portion of the message in subsequent scans. In this way, the determination of the ladder scanning task is maintained. That is, the scan speed of the ladder scan is not affected by the execution of the integrated web server.
[0024]
It is understood that the above-described configuration is merely an example to which the principle of the present invention is applied. Numerous modifications and alternative arrangements can be devised by those skilled in the art without departing from the spirit and scope of the invention. It is intended that the appended claims cover such modifications and arrangements.
[Brief description of the drawings]
FIG.
FIG. 4 is a block diagram illustrating various logic components of a PLC having an integrated web server according to the present invention.
FIG. 2
FIG. 3 is a block diagram showing several PLCs transmitting data to a remote computer.
FIG. 3
5 is a flow chart showing the main steps in the routine operation of a PLC having an integrated web server according to the present invention.

Claims (18)

A controller for use as part of an industrial control system or as part of an automation system, said controller controlling at least one input / output device, said controller being a computer located at a remote location through a network such as the Internet. And two-way processing,
a) means for performing a controller scan including solving ladder logic associated with the at least one controlled input / output device, wherein the controller scan has a predetermined scan speed;
b) a web server for collecting data related to the control function, the web server responding to the request for the data from the remote computer;
c) a web page comprising data and associated searchable instructions, the web page being executable by the remote computer, the instructions displaying the data in a predetermined format on the remote computer; Web page
With
A controller in response to the data request, wherein the web server serves one or more web pages and displays the requested data therein. The controller of claim 1, wherein the means for collecting data executes as a background task. The controller of claim 1, wherein the means for collecting data executes in parallel with the means for performing controller scanning. The controller of claim 1 wherein said means for collecting data in response to a data request from said remote computer comprises less than a predetermined maximum number of bytes and instructions per scan by said means for performing said controller scan. Controller that supplies a combination of 5. The controller of claim 4, wherein the maximum number of bytes per scan is communicable to the network for delivery to the remote computer in less than the time required for a single scan by the means for performing the controller scan. A controller that is predetermined to the number of bytes. The controller of claim 1, wherein at least a portion of the data resides on a different controller, and wherein the data collection means is configured to access the different controller via a computer network to obtain the data. . The controller of claim 1, wherein the controller is further configured to customize a transmission control program (TCP) / Internet Protocol (IP) stack to convey any MODBUS command or message as a single TCP / IP packet. Controller. The controller of claim 7, wherein the web server comprises a file server that serves the web page. 9. The controller of claim 8, wherein the web server further comprises a hypertext transfer protocol (THHP) server with file retrieval functionality. 10. The controller of claim 9, wherein the web server further receives new web pages or exchange web pages over the network via the TCP / IP stack and supplies them to the file server. A controller with a transfer protocol (FTP) server. 8. The controller of claim 7, wherein the means for performing a controller scan is coupled to a MODBUS handler, and further comprising interfacing the MODBUS handler to the TCP / IP stack to provide the means for performing a controller scan to the network. Controller to join. The controller of claim 7, wherein said means for performing said controller scan is coupled to a local network by coupling said means for performing said controller scan to a local network input / output interface. A method for controlling at least one input / output device of an industrial control system or an automation system and performing interactive processing with a computer located at a remote place through a network such as the Internet,
a) performing a controller scan that includes solving ladder logic associated with the at least one controlled input / output device, wherein the controller scan has a predetermined scan speed;
b) collecting data related to the control function;
c) storing a web page consisting of data and related instructions retrievable and executable by the remote computer, wherein the instructions are stored on the remote computer in a predetermined format. Displaying data, steps,
d) responding to a request for the data from the remotely located computer and providing one or more of the web pages used to display the requested data;
Consisting of: 14. The method of claim 13, wherein the steps of collecting data and responding to a request for the data are performed as background tasks. 14. The method of claim 13, wherein the step of collecting data and the step of responding to a request for the data are performed in parallel with the step of performing the controller scan. 14. The method of claim 13, wherein in response to a data request from the remote computer, providing less than a predetermined maximum number of bytes of data and instructions per scan. 17. The method of claim 16, wherein the maximum number of bytes per scan is predetermined to be the number of bytes that can be communicated to the network for delivery to the remote computer in less than the time required for a single scan. . 14. The method of claim 13, further comprising transmitting a MODBUS command or message over the network, wherein any MODBUS command or message is transmitted over the network as a single TCP / IP package.

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